damp with the heat of
the tropics which is so destructive to perennial plants from a temperate
climate. On the other hand, the most humid and hottest districts will have
afforded an asylum to the tropical natives. The mountain-ranges north-west
of the Himalaya, and the long line of the Cordillera, seem to have afforded
two great lines of invasion: and it is a striking fact, lately communicated
to me by Dr. Hooker, that all the flowering plants, about forty-six in
number, common to Tierra del Fuego and to Europe still exist in North
America, which must have lain on the line of march. But I do not doubt that
some temperate productions entered and crossed even the _lowlands_ of the
tropics at the period when the cold was most intense,--when arctic forms
had migrated some twenty-five degrees of latitude from their native country
and covered the land at the foot of the Pyrenees. At this period of extreme
cold, I believe that the climate under the equator at the level of the sea
was about the same with that now felt there at the height of six or seven
thousand feet. During this the coldest period, I suppose that large spaces
of the tropical lowlands were clothed with a mingled tropical and temperate
vegetation, like that now growing with strange luxuriance at the base of
the Himalaya, as graphically described by Hooker.

Thus, as I believe, a considerable number of plants, a few terrestrial
animals, and some marine productions, migrated during the Glacial period
from the northern and southern temperate zones into the intertropical
regions, and some even crossed the equator. As the warmth returned, these
temperate forms would naturally ascend the higher mountains, being
exterminated on the {379} lowlands; those which had not reached the equator
would re-migrate northward or southward towards their former homes; but the
forms, chiefly northern, which had crossed the equator, would travel still
further from their homes into the more temperate latitudes of the opposite
hemisphere. Although we have reason to believe from geological evidence
that the whole body of arctic shells underwent scarcely any modification
during their long southern migration and re-migration northward, the case
may have been wholly different with those intruding forms which settled
themselves on the intertropical mountains, and in the southern hemisphere.
These being surrounded by strangers will have had to compete with many new
forms of life; and it is proba